1.1. Technical Field
Pressure-sensitive adhesives are adhesives which, in the dry state, are permanently adhesive at room temperature and adhere firmly to various surfaces by simple contact under a slight manual pressure.
Pressure-sensitive adhesives are generally characterized by the following three properties: the adhesive power, the peel strength and the shear strength. The first property reflects the ability of the adhesive to rapidly adhere; the second defines the ability to withstand withdrawal by peeling; and the third characterizes the ability of the adhesive to maintain its initial position when it is subjected to shear forces.
In the field of industrial adhesion, it is generally necessary to specifically adapt the adhesive to the materials to be assembled and to the uses which are made of them. This is obtained by suitably balancing the above-mentioned properties (adhesive power, peel strength and shear strength) as a function of the desired aim.
1.2. Description of Related Art
Acrylic polymers constitute a particularly advantageous class of compounds for the preparation of pressure-sensitive adhesives. They are transparent and colorless, do not yellow under the influence of solar radiation and exhibit good resistance to oxidation. In addition, some of these polymers exhibit intrinsic adhesive properties generally superior to most other polymers.
Pressure-sensitive adhesives based on acrylic polymers, namely polyacrylates and polymethacrylates, are widely used in the field of labelling, in particular for producing self-adhesive labels and tapes. Poly(meth)-acrylates are generally applied in solution in an organic solvent or in the form of an aqueous emulsion.
For some years, increasing interest has been directed at aqueous emulsions, which exhibit certain advantages:
they are incombustible; PA1 they do not contain any solvent capable of escaping into the atmosphere; PA1 their adhesive nature can be reinforced by the introduction of a tackifying agent; PA1 they exhibit a high solids content (generally greater than 50% by weight), good resistance to ageing and great ease of use. PA1 R.sub.1, R.sub.2 and R.sub.3, which are identical or different, represent a linear or branched C.sub.1 -C.sub.8, preferably C.sub.2 -C.sub.6, alkyl radical and PA1 R.sub.4 represents H or an acyl radical COR.sub.5 wherein R.sub.5 is a linear or branched C.sub.1 -C.sub.3 alkyl radical. PA1 85 to 98% by weight of alkyl (meth)acrylate PA1 1 to 5% by weight of polar copolymerizable monomer PA1 1 to 10% by weight of polyunsaturated copolymerizable monomer. PA1 90 to 95% by weight of alkyl (meth)acrylate PA1 2 to 4% by weight of polar copolymerizable monomer PA1 3 to 6% by weight of polyunsaturated copolymerizable monomer. PA1 acids or bases which, by modifying the pH, make possible good compatibility with other emulsions and/or additives. Mention may be made, by way of example, of aqueous ammonia solutions or sodium hydroxide, PA1 coalescence agents which improve the ability to form a film, PA1 fillers, such as silica, calcium carbonate or zinc oxide, and/or pigments, PA1 plasticizers which modify the rheology and the properties of the adhesive. Mention may be made, by way of example, of monomeric phthalates, such as dioctyl phthalate, di-n-butyl phthalate or diisodecyl phthalate, dibenzoates, polyadipates, poly(propylene glycol) alkylphenyl ethers, and chlorinated paraffins, PA1 conventional tackifying resins, such as natural resins, unmodified or modified, and hydrocarbon resins derived from petroleum, PA1 foam-suppressants, such as heavy alcohols and their derivatives, PA1 and bactericides and/or fungicides, such as formaldehyde and its derivatives, benzimidazoles, iso-thiazolones and chloroacetamides.
These emulsions are obtained by emulsion polymerization in water of (meth)acrylic monomers, with which is generally combined a tackifying agent, such as a natural resin or a resin of petroleum origin, or an elastomer.
The latter compounds can be incorporated by prior dissolution in a hydrocarbon solvent, followed by an addition of water to form an emulsion and by the removal of the solvent. This way of operating exhibits, however, the disadvantage of allowing a not insignificant residual amount of solvent to remain in the final emulsion.
The most common incorporation consists of dissolving the tackifying agent or the elastomer in the acrylic monomers before carrying out the polymerization reaction. Numerous documents report such a way of operating:
For example, in U.S. Pat. No. 5,315,055 and WO 90/15111, a pressure-sensitive adhesive is obtained for which the peel force is increased by dissolving, in acrylic monomers, a tackifying resin exhibiting a molecular weight of between 500 and 5000 and an aromatic content of at least 10% by weight.
In JP-A-08-231937 the introduction of 5 to 30 parts by weight of a tackifying resin and of a polymerizable anionic emulsifier into 100 parts by weight of acrylic monomers makes it possible to obtain a self-adhesive which adheres to polytetrafluorethylene (PTFE).
In JP-A-54-023641, a permanent adhesive which can adhere to varied substrates is prepared by dissolving a compound of high molecular weight in a mixture of acrylic monomers, of copolymerizable monomers, and of an emulsifier. The compound of high molecular weight is a tackifying resin or an elastomer.
In JP-A-58-185668, a terpene-phenol or terpene-alkyl phenol resin is continuously added to a mixture of acrylic monomers and of copolymerizable monomers, optionally in the presence of an emulsifier, to form an aqueous emulsion having in particular good resistance to water.
Finally, in U.S. Pat. No. 4,939,190, provision is made to prepare pressure-sensitive adhesives having the ability to adhere to polyolefins. The process, in two stages, consists in polymerizing an ethylenically unsaturated monomer in the presence of a tackifying resin, in order to form an emulsion in which the particles are composed of a resin core and of an outer polyacrylate layer, and subjecting the said emulsion to a polymerization in the presence of an ethylenically unsaturated monomer. The emulsion obtained is composed of particles having a multilayer structure.
The pronounced adhesive nature of the adhesive emulsions which have just been mentioned proves to be a major disadvantage for applications involving labelling. These adhesives can be described as permanent self-adhesives, that is to say that it is difficult to remove them without part of the adhesive, indeed even the label or the tape, remaining on the substrate to which they have been affixed.